Stocking with eggs has been widely used as a management measure to support degraded salmonid stoc... more Stocking with eggs has been widely used as a management measure to support degraded salmonid stocks. In Finland, Atlantic salmon and both sea migrating and lake-migrating brown trout are stocked as eggs, alevins, fry, parr, and smolt, whereas trout are also stocked as mature fish. The aim of this stocking is to improve catches and to support collapsed spawning stocks.We assessed the success of stocking with brown trout eggs in a study of 17 Finnish boreal forest rivers, of which 9 were subject to egg stocking. All rivers contained some naturally spawning trout. In 16 rivers, including non-stocking years and unstocked rivers, egg stocking did not increase the total (wild and stocked) density of 0-year-old parr. However, those rivers with higher existing trout densities in non-stocking years seemed to benefit most from stocking, suggesting some role of river-specific extrinsic factors affecting egg-to-parr survival. In one river monitored for 14 years, only a weak correlation was foun...
ABSTRACT We investigated differences in the abundance, community composition and taxon richness o... more ABSTRACT We investigated differences in the abundance, community composition and taxon richness of sublittoral macroinvertebrates in boreal lakes containing introduced invasive signal crayfish (Pacifastacus leniusculus) or native noble crayfish (Astacus astacus). We hypothesised that sublittoral macroinvertebrate assemblages in lakes containing noble crayfish, which reportedly prefer the shallow littoral, would differ little from those in lakes without crayfish, in contrast to assemblages in lakes containing signal crayfish, which reportedly forage at greater depths. In a set of 24 small and medium-sized Finnish lakes, eight lakes contained signal crayfish, eight had noble crayfish and eight control lakes had no crayfish. At three sites per lake, we sampled macroinvertebrates quantitatively from soft sediment in the sublittoral. The sublittoral macroinvertebrate assemblage in lakes with noble crayfish and control lakes was similar. In contrast, lakes with signal crayfish had a lower density of Trichoptera and Ephemeroptera, as well as a lower density and species richness of Chironomidae, and a lower overall benthic density and taxon richness. In contrast to some previous reports from other habitats, we infer that introduced signal crayfish have stronger negative effects than native noble crayfish on sublittoral macroinvertebrates. Hence, the ecological equivalence or differences of crayfish species cannot be generalised across habitats.
The character and magnitude of predation by the invasive, ectothermic Pacifastacus leniusculus, a... more The character and magnitude of predation by the invasive, ectothermic Pacifastacus leniusculus, a crayfish widely introduced to Europe and Japan from North America, on the eggs of coregonid fishes, vendace Coregonus albula and whitefish Coregonus lavaretus were examined by experimentation, modelling and field data. The present results showed that P. leniusculus has the potential to be very efficient predator of fish eggs under winter conditions, but the predation by P. leniusculus did not significantly decrease production of coregonid larvae during the years with a high P. leniusculus population in the study lake. Hence, the mortality caused by the novel invertebrate predator appeared to compensate for other yet unexplored mortality factors instead of having an additive effect on the present salmonids.
ABSTRACT Invasive crayfish are widely acknowledged to have negative effects on benthic food webs ... more ABSTRACT Invasive crayfish are widely acknowledged to have negative effects on benthic food webs in lakes, but few studies have investigated such effects at wider spatial scales and in varying habitats under natural conditions. We examined the effects of introduced signal crayfish (Pacifastacus leniusculus) on the macroinvertebrate assemblages of different habitats in two large boreal lakes. We evaluated whether the density, taxon richness and species composition are altered by the non-native crayfish and whether the responses are similar for stony and vegetated habitats and across a depth gradient. We also studied the influence of crayfish on periphyton biomass at stony sites, as a potential link to changes in macroinvertebrate communities. In both lakes, macroinvertebrate density was similar between crayfish and non-crayfish sites across the habitats and depths studied. However, macroinvertebrate taxon richness was significantly lower, and community composition was altered in the presence of crayfish at stony sites. No similar pattern was detected at vegetated sites or in deeper sublittoral areas. The amount of periphyton was similar regardless of the presence of crayfish, and no clear direct or indirect crayfish–periphyton interaction was detected. Our results suggest that introduced signal crayfish can have negative effects on the littoral macroinvertebrates of large boreal lakes, but that these effects are habitat specific. Our findings highlight how the evaluation of possible effects of invasive species needs to be carried out comprehensively across different habitats and spatial scales if conclusions are to be robust.
Stocking with eggs has been widely used as a management measure to support degraded salmonid stoc... more Stocking with eggs has been widely used as a management measure to support degraded salmonid stocks. In Finland, Atlantic salmon and both sea migrating and lake-migrating brown trout are stocked as eggs, alevins, fry, parr, and smolt, whereas trout are also stocked as mature fish. The aim of this stocking is to improve catches and to support collapsed spawning stocks.We assessed the success of stocking with brown trout eggs in a study of 17 Finnish boreal forest rivers, of which 9 were subject to egg stocking. All rivers contained some naturally spawning trout. In 16 rivers, including non-stocking years and unstocked rivers, egg stocking did not increase the total (wild and stocked) density of 0-year-old parr. However, those rivers with higher existing trout densities in non-stocking years seemed to benefit most from stocking, suggesting some role of river-specific extrinsic factors affecting egg-to-parr survival. In one river monitored for 14 years, only a weak correlation was foun...
ABSTRACT We investigated differences in the abundance, community composition and taxon richness o... more ABSTRACT We investigated differences in the abundance, community composition and taxon richness of sublittoral macroinvertebrates in boreal lakes containing introduced invasive signal crayfish (Pacifastacus leniusculus) or native noble crayfish (Astacus astacus). We hypothesised that sublittoral macroinvertebrate assemblages in lakes containing noble crayfish, which reportedly prefer the shallow littoral, would differ little from those in lakes without crayfish, in contrast to assemblages in lakes containing signal crayfish, which reportedly forage at greater depths. In a set of 24 small and medium-sized Finnish lakes, eight lakes contained signal crayfish, eight had noble crayfish and eight control lakes had no crayfish. At three sites per lake, we sampled macroinvertebrates quantitatively from soft sediment in the sublittoral. The sublittoral macroinvertebrate assemblage in lakes with noble crayfish and control lakes was similar. In contrast, lakes with signal crayfish had a lower density of Trichoptera and Ephemeroptera, as well as a lower density and species richness of Chironomidae, and a lower overall benthic density and taxon richness. In contrast to some previous reports from other habitats, we infer that introduced signal crayfish have stronger negative effects than native noble crayfish on sublittoral macroinvertebrates. Hence, the ecological equivalence or differences of crayfish species cannot be generalised across habitats.
The character and magnitude of predation by the invasive, ectothermic Pacifastacus leniusculus, a... more The character and magnitude of predation by the invasive, ectothermic Pacifastacus leniusculus, a crayfish widely introduced to Europe and Japan from North America, on the eggs of coregonid fishes, vendace Coregonus albula and whitefish Coregonus lavaretus were examined by experimentation, modelling and field data. The present results showed that P. leniusculus has the potential to be very efficient predator of fish eggs under winter conditions, but the predation by P. leniusculus did not significantly decrease production of coregonid larvae during the years with a high P. leniusculus population in the study lake. Hence, the mortality caused by the novel invertebrate predator appeared to compensate for other yet unexplored mortality factors instead of having an additive effect on the present salmonids.
ABSTRACT Invasive crayfish are widely acknowledged to have negative effects on benthic food webs ... more ABSTRACT Invasive crayfish are widely acknowledged to have negative effects on benthic food webs in lakes, but few studies have investigated such effects at wider spatial scales and in varying habitats under natural conditions. We examined the effects of introduced signal crayfish (Pacifastacus leniusculus) on the macroinvertebrate assemblages of different habitats in two large boreal lakes. We evaluated whether the density, taxon richness and species composition are altered by the non-native crayfish and whether the responses are similar for stony and vegetated habitats and across a depth gradient. We also studied the influence of crayfish on periphyton biomass at stony sites, as a potential link to changes in macroinvertebrate communities. In both lakes, macroinvertebrate density was similar between crayfish and non-crayfish sites across the habitats and depths studied. However, macroinvertebrate taxon richness was significantly lower, and community composition was altered in the presence of crayfish at stony sites. No similar pattern was detected at vegetated sites or in deeper sublittoral areas. The amount of periphyton was similar regardless of the presence of crayfish, and no clear direct or indirect crayfish–periphyton interaction was detected. Our results suggest that introduced signal crayfish can have negative effects on the littoral macroinvertebrates of large boreal lakes, but that these effects are habitat specific. Our findings highlight how the evaluation of possible effects of invasive species needs to be carried out comprehensively across different habitats and spatial scales if conclusions are to be robust.
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Papers by Timo J. Ruokonen